The base unit of a DOMString. This indicates that
indexing on a DOMString occurs in units of 16 bits.
This must not be misunderstood to mean that a DOMString
can store arbitrary 16-bit units. A DOMString is a
character string encoded in UTF-16; this means that the restrictions
of UTF-16 as well as the other relevant restrictions on character strings
must be maintained. A single character, for example in the form of a
numeric character reference, may correspond to one or two 16-bit units.

A read only node is a node that is immutable. This
means its list of children, its content, and its attributes, when it
is an element, cannot be changed in any way. However, a read only node
can possibly be moved, when it is not itself contained in a read only
node.

A node is a well-formed XML node if its
serialized form, without doing any transformation during its
serialization, matches its respective production in [XML 1.0] or [XML 1.1] (depending on the XML
version in use) with all well-formedness constraints related
to that production, and if the entities which are referenced
within the node are also well-formed. If namespaces for XML
are in use, the node must also be namespace
well-formed.